Simplex were the cheapest and lighteset derailleurs on the market by virtue of thier injection molded plastic constructiom. They were the most effective way to keep the cost and weight of your bicycle down. I believe they also had a factory in Italy.

As for the Juventus brand, intitially I thought it may be Carnielli, who made Bottechia, as there is a big C on the headbadge and they had a habit of putting their big race victories on the decals. However, those dates on the stickers correspond to victories for Magni, LeJeune and Peugeot. Now Magni were manufactured by Chiorda, so I suspect this is the source of the brand. It also explains the C on the headbadge. I'm not sure what the tie with LeJeune and Peugeot is.